When transferring fluid (e.g., a liquid or gas) through a transfer apparatus (e.g., a pipe or a tube), the volume of fluid transferred may be measured through the use of resistive thermal sensors that are in thermal contact with the transfer apparatus. These resistive thermal sensors are often lengths of wire (that are coiled around the transfer apparatus) whose resistance value varies based on the temperature of the sensor. Typically, two sensors are positioned along the length of the transfer apparatus (i.e., one upstream and one downstream) and a constant current is driven through each thermal sensor, resulting in ohmic heating of the upstream and downstream thermal sensors. The heat generated by the upstream sensor is transferred into the fluid within the transfer apparatus. If this fluid is moving, this heat is transferred downstream (via the fluid), resulting in the heating of the downstream sensor. Accordingly, when there is no fluid flowing within the transfer apparatus, heat is not transferred between the upstream and downstream sensors and the sensors remain at an equal temperature. However, as the flow of fluid within the transfer apparatus increases, the amount of heat transferred from the upstream sensor to the downstream sensor also increases.
As the resistance of the thermal sensors varies in accordance with the temperature of the sensors, by monitoring the resistance of these sensors, the volume of fluid flowing within the transfer apparatus may be determined. As there is often a constant current flowing through these thermal sensors, the resistance values of the thermal sensors are often measured by monitoring the voltage potential across each of the thermal sensors. Unfortunately, it is often difficult to ensure that the current flowing through each thermal sensor is equal, and variations in this current induce inaccuracies in the measurement system.